Multiple antenna multi-frequency measurement system

ABSTRACT

A measurement system and an associated method for determining the positions of multiple antennas to centimeter level accuracy. A plurality of first filters, one filter for each antenna of a plurality of antennas, are each operable to obtain information from a respective one of the plurality of antennas at a primary frequency. A second filter connects with at least one of the plurality of antennas. A processor calibrates biases for the information at the primary frequency with a RF path bias calibration algorithm as a function of an output of the second filter.

REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.10/789,868, filed Feb. 27, 2004, now U.S. Pat. No. 7,138,944, which is acontinuation of PCT Application Serial No. PCT/US02/36960, filed Nov.19, 2002 (designating the United States and published in English), whichclaims priority to U.S. Provisional patent application Ser. No.60/332,278 filed Nov. 20, 2001, the entire disclosures of which areincorporated herein by reference.

BACKGROUND

The present invention relates to GPS measurement systems, and moreparticularly with a measurement system and a method for tracking anobject using a number of multi-frequency antennas.

The Global Positioning System (GPS) is a satellite based navigationsystem having a constellation of 24 Earth orbiting satellites. Thesesatellites are approximately uniformly dispersed around six circularorbits having four satellites each. Theoretically, four or more GPSsatellites are visible from most points on the Earth's surface.

Each GPS satellite presently transmits at two frequencies: L1 (1575.42MHz) and L2 (1227.60 MHz). There exists provision (for the future) for athird frequency L5 (1176.45 MHz) as well. The L1 frequency has twodifferent spread-spectrum codes modulated on it: a coarse acquisition(C/A) code and a Y code. The C/A code is an unclassified code intendedfor civilian navigation. It has a chipping rate of 1.023 MHz and asequence length of 1023 chips. The Y code is a classified unknown code;people doing research in this area have found it to be the product oftwo codes: a precise (P) code and a W code. The P code is anunclassified code with a chipping rate of 10.23 MHz. The P code is longenough that it does not repeat during a week; it is reset at thebeginning of the GPS week for each satellite. The P code is mixed withthe classified W code to get an encrypted Y code. The W code has beenempirically found to have a chipping rate of approximately 500 KHz. TheY code is modulated onto the L1 carrier in quadrature with the C/A codeand with half the power of the C/A code. The Y code is also modulatedonto the L2 carrier signal with half the power of L1 Y code. Both C/Aand P codes are unique for each satellite.

GPS receivers are commonly used for a variety of applications involvingtracking of the position of various objects. The object to be tracked iscoupled to one or more GPS antennas that receive signals from the GPSsatellites. Depending upon the level of accuracy and response timedesired by a user, an appropriate method of obtaining position of anobject using GPS can be adopted.

A commonly used method that yields the position information (withinmeters) is the pseudorange method. This method utilizes the C/A codeand/or the P code modulated onto the carrier signals from the GPSsatellites.

Use of a reference antenna that employs carrier phase measurements andhas known coordinates can further enhance the accuracy of positiondetermination of the antennas. Differential carrier phase GPSmeasurement is a technique which determines the position of a givenantenna with respect to a reference antenna. The other antennas, knownas roving antennas, are free to roam around. Measurements of the carrierphase at the reference antenna and the roving antennas are used tocalculate the relative position of the antennas to centimeter levelaccuracy. Before the carrier phase measurements can be used fordetermining position accurately, the carrier cycle ambiguity or thenumber of complete carrier cycles between the antennas (referenceantenna and roving antennas) must be determined.

Typically, the conventional approach for resolving carrier cycleambiguities starts with a code-based differential GPS solution.Thereafter, the integer count for all the L1 satellite signals used inthe position solution is determined. The integer solution is oftenambiguous due to errors induced by receiver noise and multipath for bothcode and carrier based measurements. The integer solution is averagedover a period of time to converge on the exact solution. This processbenefits from the intervening satellite motion. However, the process maytake from a single measurement to several minutes worth of data to yieldthe correct integers depending on the number of satellites, the qualityof the phase measurements and the desired level of confidence.

Dual frequency receivers that utilize both L1 and L2 frequency signalscan determine carrier cycle ambiguities much faster than a singlefrequency receiver. A technique that uses both L1 and L2 phasemeasurements is faster than the one using just L1 carrier phasemeasurements. The phase of the L2 carrier is used to assist in resolvingthe carrier cycle ambiguity of the L1 signals.

The L1 carrier can be recovered by using a standard code correlationtechnique as the C/A code is known for each of the satellites. The L2carrier signal is encrypted, thus only military GPS receivers that areaware of the encryption key can reconstruct the L2 signal with highaccuracy. Civilian receivers can also reconstruct the L2 carrier signalusing any of the known standard techniques, most of which derive the L2carrier using the L1 carrier. However, the signal to noise ratio (SNR)of resulting L2 signal is lower than L2 signals reconstructed usingmilitary receivers.

Typically, GPS receivers employ dedicated RF sections for both the L1and the L2 frequencies for every antenna to be tracked. The RF sectiondown converts L1 and/or L2 RF signal and samples the signals for furtherprocessing. However, it may be prohibitively expensive to have adedicated L1/L2 RF section for each antenna to be tracked. Moreover, inapplications where it is desirable to track the position of multiple GPSantennas on a moving platform, the resulting system becomes very bulky.GPS receivers used for such applications usually employ multiplexing ofboth L1 as well as L2 signals to reduce the hardware cost. A patent thatrefers to such a GPS receiver is U.S. Pat. No. 6,154,170 titled‘Enhanced Attitude Determination System Using Satellite NavigationReceiver With Antenna Multiplexing’, granted to Trimble NavigationLimited, Sunnyvale, Calif. Yet another patent that describes a systemthat multiplexes RF signals for multiple antennas is U.S. Pat. No.5,917,448 titled ‘Attitude Determination System With Sequencing AntennaInputs’, granted to Rockwell Science Center Inc. of Thousand Oaks,Calif.

Although, some of the abovementioned patents do refer to multiplexingfor reducing the hardware cost, the SNR for L1/L2 measurements is low asthe RF sections receive signals only for a fraction of the time. Hence,there exists a need for a system that derives L1/L2 signals with a highSNR and low hardware cost.

SUMMARY

The present invention is directed to a measurement system that satisfiesthe need for a system that tracks an object with very little incrementalhardware cost per additional antenna, and at the same time improves theSNR of the carrier phase of a primary frequency RF signal.

One object of the present invention is to calibrate slowly changing linebiases in the primary frequency RF sections.

Some embodiments provide for a measurement system for tracking positionof multiple antennas to centimeter level accuracy. The system requiresvery little incremental hardware cost per additional antenna to betracked. Carrier phase of the signals received from a satellite are usedto determine relative positions of the antennas. A splitter dedicated toeach antenna to be tracked splits the radio frequency signal receivedfrom the satellite into at least two output signals.

Thereafter, primary frequency RF signals are filtered, down convertedand sampled by a primary frequency RF section dedicated to each antenna.The primary frequency RF section outputs sampled primary signals.Secondary frequency RF signals from all antennas are multiplexed andinput to a secondary frequency RF section corresponding to eachsecondary frequency. The secondary frequency RF section filters, downconverts and samples multiplexed secondary frequency RF signals tooutput sampled secondary signals. A correlator then derives code andcarrier phase measurements for the primary and secondary frequency RFsignals. A processor extrapolates the sampled secondary signals andresolves carrier cycle ambiguities for the sampled primary signals usingthe carrier phase of the sampled secondary signals. Finally, the phaseof the primary signals is used to determine relative positions of theantennas. Using the position of the antennas, the object is tracked.

BRIEF DESCRIPTION OF THE DRAWINGS

The preferred embodiments of the invention will hereinafter be describedin conjunction with the appended drawings provided to illustrate and notto limit the invention, wherein like designations denote like elements,and in which:

FIG. 1 is a block diagram of the measurement system in accordance withthe preferred embodiment of the present invention; and

FIG. 2 is a block diagram of the measurement system in accordance withan alternate embodiment of the present invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

Overview of the Invention

The present invention provides a measurement system and an associatedmethod for ‘tracking an object’ (or a number of objects). The phrase‘tracking an object’, means finding position, and sometimes velocity,time, attitude and angular velocity associated with the object as well.For the purpose of tracking the object, the object is coupled to themeasurement system. The measurement system comprises a number ofantennas that are connected (or affixed) to the object so that theobject may be tracked. For some applications, it may be desirable totrack the position of an object with respect to the position of anotherobject. For example, in case of a moving platform such as a farm tractorpulling a field implement, the antennas may be mounted both on the farmtractor as well as the field implement. Here, it is desirable to knowthe position of the antenna mounted on the tractor relative to theposition of the antenna mounted on the field implement. All the antennasreceive radio frequency (RF) signals from a plurality of (two or more)signal generating sources: satellites or pseudolite. A pseudolite(pseudo satellite) is a low power transmitter that transmits an RFsignal, most commonly at the L1 RF frequency. Typically, a pseudolitetransmits signals at a higher power as compared to GPS satellites for ashorter range.

The RF signal that is received by the antenna comprises a primaryfrequency RF signal and at least one secondary frequency RF signal. Ifthere are more than one secondary frequency RF signals, each secondaryfrequency RF signal propagates through space at a different frequency.

Each of these RF signals, upon reception by the antenna, is passedthrough an RF splitter. The RF splitter splits the RF signals into twooutput signals: a first output signal and a second output signal. Boththe first output signal and the second output signal include the primaryfrequency RF signal and all the secondary frequency RF signals. Thenumber of antennas and RF splitters is same with a one-to-one connectionbetween the two i.e. each antenna is connected to a different RFsplitter.

The first output signal from each of the RF splitters is sent to aprimary frequency RF section. Each primary frequency RF section isconnected to a different antenna. The primary frequency RF sectionprocesses the first output signal to output a sampled primary signal.This processing involves the following steps: filtering of the firstoutput signal to obtain the primary frequency RF signal; down convertingof the primary frequency RF signal; and sampling of the primaryfrequency RF signal. Just as there is a different RF splitter connectedto each of the antennas, there is a different primary frequency RFsection connected to each of the RF splitters i.e. the number of theprimary frequency RF sections is equal to the number of the RFsplitters.

The second output signal from each of the RF splitters is sent to amultiplexer. The function of the multiplexer is to switch between thesecond output signals to output one or more (at least one) multiplexedsignals. The multiplexer is connected to all the RF splitters such thatinput to the multiplexer comprises the second output signal from each ofthe RF splitters i.e. the number of inputs of the multiplexer is equalto the number of the RF splitters, which in turn is equal to the numberof the antennas.

Each of the multiplexed signals is sent to a different secondaryfrequency RF section. Each secondary frequency RF section operates at asecondary frequency; although the secondary frequency for differentsecondary frequency RF sections need not be different always i.e. it ispossible that more than one secondary frequency RF sections operate forthe same secondary frequency. All the secondary frequency RF sectionsare connected to the output of the multiplexer. The function of thesecondary frequency RF section is the same as that of the primaryfrequency RF section i.e. the secondary frequency RF section processesthe multiplexed second output signal to output a sampled secondarysignal. This processing involves the steps of filtering of themultiplexed second output signal to obtain the secondary frequency RFsignal, and down converting and sampling of the secondary frequency RFsignal.

The number of the secondary frequency RF sections is dependent upon thenumber of secondary frequencies in use. A possibility is that the numberof secondary frequency RF sections is the same as the number ofsecondary frequencies in use i.e. there is just one secondary frequencyRF section for each of the secondary frequencies with each of thesecondary frequency RF sections operating for the correspondingsecondary frequency.

Another possibility is that the number of secondary frequency RFsections is more than the number of secondary frequencies in use. Inthis case, there will be more than one secondary frequency RF sectionoperating for the same secondary frequency. This implies the following:the secondary frequency RF signals (for the same secondary frequency)received by more than one antenna will be processed simultaneously (i.e.in parallel). This improves the accuracy of the measurement system.

The primary sampled signals (output by the primary frequency RFsections) and the secondary sampled signals (output by the secondaryfrequency RF sections) are input to a correlator. The correlator isconnected to all the primary frequency RF sections as well as all thesecondary frequency RF sections. The correlator derives the code andcarrier phase for the primary frequency RF signals by correlating thesampled primary signals. Because of multiplexing, the secondaryfrequency RF signals from all the antennas are not available to thecorrelator at all times. Hence, the correlator derives initial code andinitial carrier phase for the secondary frequency RF signals by onlycorrelating the sampled secondary signals.

All the code and carrier phase information generated by the correlatoris passed over to a processor. The processor reconstructs the carrierphase of the secondary frequency RF signal using the initial carrierphase of the secondary frequency RF signal and the carrier phase of theprimary frequency RF signal. This is done by extrapolating the carrierphase of the secondary frequency RF signal from the initial carrierphase using the fact that both the primary frequency RF signal and thesecondary frequency RF signal are generated using the same clock in thesatellite; because of this, there is a constant frequency ratio betweenthe primary frequency RF signal and the secondary frequency RF signal.

Thereafter, the processor resolves carrier cycle ambiguities for theprimary frequency RF signal using the carrier phase of the primaryfrequency RF signal and the carrier phase of the secondary frequency RFsignal for all pairs of antennas in the system.

A method comprising the above-mentioned steps of reconstructing carrierphase and resolving carrier cycle ambiguities is hereinafter referred toas an intra-platform parallel processing algorithm. This algorithm isdescribed in detail below.

The carrier phase for the secondary frequency RF signal is extrapolatedusing the following formula:φ₂(t)=φ₂(t ₀)+(φ₁(t)−φ₁(t ₀))*(f ₂ /f ₁)  (1)

where, φ₂ is the carrier phase of the secondary frequency RF signal;

φ₁ is the carrier phase of the primary frequency RF signal;

f₂ is the frequency of the secondary frequency RF signal;

f₁ is the frequency of the primary frequency RF signal;

t₀ is the instant of time when the carrier phase of the secondaryfrequency RF signal was last measured; and

t is the time at which the carrier phase of the secondary frequency RFsignal is to be determined.

When the RF signals leave the satellite, the ratio of frequencies ofprimary and secondary frequency RF signals is equal to f₂/f₁. But due toionospheric effects, this relation between the ratio of RF frequenciesand f₂/f₁, is not exactly true when the antennas receive the RF signals.However, it is a very good approximation over several seconds (3-10cycles over an hour is a representative figure). Moreover, the use ofthis algorithm enables the measurement system to resolve carrier cycleambiguities very quickly. This is possible since carrier phases ofsecondary frequency RF signals for one antenna can be correlated and forall the other antennas, the carrier phase can be extrapolated at anypoint of time. Therefore, it is adequate for most applications to havejust one RF section per secondary frequency.

This completes the description of the extrapolation step in theintra-platform parallel processing algorithm. The carrier cycleambiguities may be resolved using any of the methods known in the art.

After having resolved carrier cycle ambiguities, the processor obtainsthe position of the antennas using code and phase measurements for theprimary frequency RF signal. In an alternate embodiment, the code andphase measurements for the secondary frequency RF signal are also usedin determining position; the main purpose of using the secondaryfrequency RF signals is to observe ionospheric delays.

Thereafter, the processor uses positions of the antennas to track theobject i.e. it finds the position, velocity, time, attitude and angularvelocity associated with the object. This may be accomplished byemploying any of the methods known in the art.

Hereinafter, other algorithms for resolving the carrier cycle ambiguityfor the primary frequency RF signal are described in detail. Analgorithm for calibrating the RF path biases for multiple primaryfrequency RF sections is also described.

Inter-platform Parallel Processing Algorithm

In the inter-platform parallel processing algorithm, the measurementsystem is continuously receiving communication from a reference basestation. This is the meaning of ‘inter-platform’. However, in theintra-platform algorithm, there is no interaction of the measurementsystem with any reference base station.

The reconstruction step is the same as in the intra-platform parallelprocessing algorithm. In the case of inter-platform parallel processing,extrapolation of carrier phase for the secondary frequency RF signal isdone between each antenna (on the object) and the reference base stationantenna. However, in case of intra-platform parallel processing,extrapolation of carrier phase for the secondary frequency RF signal isdone between antennas that are on the object itself.

Intra-platform Sequential Processing Algorithm

Sequential processing algorithms do not involve reconstruction of thecarrier phase making them less sensitive to ionospheric effects. This isso because the ionospheric effect is different on the primary frequencyRF signal and the secondary frequency RF signal. Due to this,reconstruction of the secondary frequency RF signal using the carrierphase of the primary frequency RF signal is slightly erroneous.

In the case of the intra-platform sequential processing algorithm, morethan one secondary frequency RF section operates for the same frequency.Due to this, carrier cycle ambiguities for the secondary frequency RFsignals for a pair of antennas are resolved simultaneously.

For example, assume that two of the secondary frequency RF sectionsoperate at the same frequency. Using these two secondary frequency RFsections, resolution of carrier cycle ambiguities for the secondaryfrequency RF signal for any pair of antennas can be done. Themultiplexer may be controlled (by the processor) such that by selectinga minimal set of antenna pairs, resolution of carrier cycle ambiguitiesfor the secondary frequency RF signal for every pair of antennas iscarried out. The minimal set of antenna pairs is defined to be that setof antenna pairs, for which, if the carrier cycle ambiguities have beenresolved, the same can be resolved for all other antennas using thecarrier cycle ambiguity resolution information of just those antennasthat are in this set. By way of an example, assuming that there are 5antennas: A, B, C, D and E, it is sufficient to resolve carrier cycleambiguity for just A-B, B-C, C-D and D-E. After carrier cycle ambiguityfor these four pairs has been resolved, any other pair can be expressedas a combination of pairs from this set e.g. carrier cycle ambiguity forB-D pair may be resolved, using corresponding information for B-C andC-D.

Inter-platform Sequential Processing Algorithm

This algorithm is similar to the intra-platform sequential processingalgorithm in the following aspect: the multiplexer keeps switching froman antenna pair to another antenna pair until carrier cycle ambiguitiesfor a minimal set of antenna pairs (or triplets, depending upon thenumber of secondary frequency RF sections operating for the samefrequency) are resolved.

The difference between the two algorithms is as follows: in theinter-platform sequential processing algorithm, just one secondaryfrequency RF section is required, whereas, in the intra-platformsequential processing algorithm, at least two secondary frequency RFsections are required. This is because the measurement system iscontinuously receiving communication from the reference base stationthat makes up for the other secondary frequency RF section.

The minimal set in this case comprises the following: an antenna of thereference base station and a first antenna of the measurement system,the antenna of the reference base station and a second antenna of themeasurement system, and so on. Once carrier cycle ambiguities for allsuch pairs have been resolved, the algorithm resolves carrier cycleambiguities for any pair of antennas of the measurement system using theresolution information for carrier cycle ambiguities for the minimal setof antenna pairs.

In a variation of the inter-platform sequential processing algorithm,the reference base station has a plurality of antennas coupled to themeasurement system. In this case, the reference base station alsobenefits from the present invention. By way of an example, the referencebase station may have antennas DA, DB, DC, and the object may haveantennas VA, VB, VC, etc. Sequential use of the secondary RF sectionswitches the secondary RF sections to DA and VA, then to DB and VB, thento DC and VC, then to DA and VB, and finally to DA and VC. Once carriercycle ambiguities for these 5 pairs of antennas have been resolved, allthe information required to resolve carrier cycle ambiguities of anyother pair of antennas is available to the measurement system. Forexample, the position of VA relative to VB and the position of DBrelative to DA can be obtained by knowing the position of VA relative toDA and the position of VB relative to DB and the position of VB relativeto DA.

RF Path Bias Calibration Algorithm

The present invention provides a RF path bias calibration algorithm thatincreases the accuracy of attitude determination.

Biases that change slowly with time may exist for different RF paths ina conventional multiple antenna GPS measurement system. This is becauseof variable time spent by the RF signals in the RF sections. Inconventional systems, these delays are assumed to be constant over aperiod of time. But some L1 RF section designs have delays that dochange with time. This is undesirable for tracking the attitude becauseconventional attitude processing algorithms often assume that delay fromthe antenna to the correlator can be calibrated just once and removedfrom all the subsequent measurements. The present invention can be usedto eliminate these biases from all the measurements.

To support execution of this algorithm, the measurement system has anadditional primary frequency RF section connected to the output of themultiplexer and to the correlator. In an alternate embodiment, one ofthe secondary frequency RF sections has the additional primary frequencyRF section.

In the algorithm for calibrating RF path biases, first, the primaryfrequency RF section that needs to be calibrated is selected.Thereafter, the processor switches the multiplexer to connect theadditional primary frequency RF section to the antenna connected to theprimary frequency RF section to be calibrated. Subsequently, a firstcarrier phase for the primary frequency RF signal passing through theprimary frequency RF section is measured. Similarly, the second carrierphase for the primary frequency RF signal passing through the additionalprimary frequency RF section is measured. Thereafter, the relative pathdelay for the primary frequency RF section using the first carrier phaseand the second carrier phase is determined. All the abovementioned stepsare repeated for all the primary frequency RF sections.

A Block Diagram in Accordance with the Preferred Embodiment

Referring now primarily to FIG. 1, an exemplery block diagram of themeasurement system in accordance with the preferred embodiment of thepresent invention is illustrated. Antennas 102 a, 102 b and 102 c aremultiple frequency antennas, which receive signals at L1, L2 and L5 RFfrequencies. The L1 RF signal is the primary frequency RF signal and theL2 and the L5 RF signals are the secondary frequency RF signals.

Antennas 102 a, 102 b and 102 c receive L1, L2 and L5 signals from atleast four GPS satellites for determining their position. Variousposition tracking algorithms exist in the art for tracking positions ofthe antennas using less than four satellites. The minimum number ofsatellites required for calculating the position of antennas 102 a, 102b and 102 c depends on the algorithm used. The present invention isapplicable with all the position tracking algorithms.

RF signals received by antenna 102 a are amplified, filtered and passedon to RF splitter 104 a connected to antenna 102 a. RF splitter 104 aseparates the power of the RF signal and outputs a first output signal106 a and a second output signal 108 a. The first output signal is theRF signal that is fed into a primary frequency RF section.

The second output signal is the RF signal that is fed to a multiplexerfor subsequent processing of secondary frequency RF signals. Antenna 102b and antenna 102 c are connected to RF splitter 104 b and RF splitter104 c, respectively. The signals output by RF splitters 104 a through104 c include L1 RF signals, L2 RF signals and L5 RF signals. Firstoutput signal 106 a is passed to L1 RF section 110 a connected to RFsplitter 104 a. L1 RF section 110 a is dedicated to the L1 RF signalsfrom antenna 102 a. L1 RF section 110 a filters first output signal 106a and receives only the L1 RF signal. Similar filtering is performed byL1 RF section 110 b and L1 RF section 110 c on first output signal 106 band first output signal 106 c, respectively to obtain the L1 RF signals.The L1 RF signal is thereafter down converted from the radio frequencyto an intermediate frequency (IF). The down conversion is performed inorder to bring down the frequency of the L1 signal from a higher radiofrequency (RF) to a lower IF for facilitating processing of the L1signals. The L1 RF signals have a frequency of 1575.42 MHz and they areusually down converted to an intermediate frequency such as 4 MHz. TheIF signals are then sampled in L1 RF section 110 a through 110 c.

Second output signal 108 a moves along a second path from RF splitter104 a to be fed to multiplexer 112 for multiplexing. Second outputsignals 108 b and 108 c are also fed to multiplexer 112 formultiplexing. The multiplexer switches between second output signal 108a, second output signal 108 b and second output signal 108 c for acontrolled fraction of time. For this purpose, a processor 116 controlsmultiplexer 112. In an alternate embodiment, multiplexer 112 switchesbetween the second output signals for a pre-configured fraction of time.Multiplexer 112 is a three input (the number of antennas) and two output(the number of secondary frequency RF sections) switch, whichmultiplexes the three incoming signals to output two signals. The twooutputs of multiplexer 112 are connected to a secondary frequency (L2)RF section 118 and another secondary frequency (L5) RF section 120.Multiplexer 112 outputs RF signals from a particular antenna, say,antenna 102 a for a fraction of time to L2 RF section 118 and L5 RFsection 120. Thereafter, multiplexer 112 switches to RF signals fromantenna 102 b. Hence, at a given instant of time, L2RF section 118 andL5 RF section 120 receive RF signals from antenna 102 a. At any time, L2RF section 118 and L5 RF section 120 receive signals from antenna 102 b.

L2 RF section 118 filters second output signal 108 a (received viamultiplexer 112) to output the L2 RF signals. L2 RF section 118 downconverts and samples the L2 RF signals thus obtained. L1 RF sections 104a through 104 c, L2 RF section 118 and L5 RF section 120 are allconnected to a clock 122 for synchronous functioning of the RF sections.

The down converted and sampled L1 RF signals from L1 RF sections 110 a,110 b and 110 c, and the down converted and sampled L2 RF signals fromL2 RF section 118 are forwarded to a correlator 124 for correlation.Correlator 124 can be embodied either as software or as hardware. Thehardware embodiment of correlator 124 includes Random Access Memory(RAM) buffers and Field Programmable Gate Arrays (FPGA). Correlator 124is implemented on one or more FPGAs depending on the number of antennasbeing tracked. Correlator 124 also consists of multiple buffer units forstoring IF samples.

Correlator 124 correlates the down converted and sampled L1 RF signalsand the L2 RF signals to derive code and carrier phases. Correlator 124outputs an in-phase and a quadrature component of the sampled RF signal.Any standard algorithm known in the art may be used for the correlationof the L1 RF signals and the L2 RF signals. Some of these algorithms aredescribed in the paper titled ‘Optimum semi-codeless carrier phasetracking of L2’, presented by K. T. Woo at The 12^(th) InternationalTechnical Meeting of the Satellite Division of the Institute ofNavigation, Nashville, Tenn., Sep. 14-17 1999. As the C/A code is knownfor the L1 RF signals, the L1 carrier and code signals may be separatedwith high-precision using methods already known in the prior art. The L2RF signals may be correlated using the relationship between the phase ofL1 RF signals and the phase of the L2 RF signals using any standardcross-correlation algorithm known in the art that reconstructs the L2carrier. For military applications, the encryption code is known and theavailable decryption information is used to correlate the L2 RF signal.

The correlated L1 RF signals and the L2 RF signals are fed to processor116 for further processing. Processor 116 resolves carrier cycleambiguities of the L1 RF signal using the intra-platform parallelprocessing algorithm.

Processor 116 runs standard loop closure algorithms as real-time tasksto track L1 code and carrier phases. Processor 116 also uses the codeand carrier phases to determine the position of each of the antennasusing standard position tracking algorithms known in the prior art.Positions of the antennas thus derived are used to track the object. Theposition of the object may be output to an external user; it may also beused by any other method for further processing.

Applications and Advantages of the Preferred Embodiment

The preferred embodiment of the present invention is applicable to anypositioning system, although exemplary embodiments of the invention havebeen illustrated with respect to the GPS. The system may also be used inother positioning systems including Global Orbiting Navigation SatelliteSystem (GLONASS), pseudolite augmented systems, or any other positioningsystem that emits RF signals for tracking an object. The object may bemobile or stationary, such as—but not limited to—a vehicle, a pole, anaircraft, a ship, a boat, or a train.

With reference to systems and/or methods employing dedicated RF sectionsfor all the RF signals from all the antennas, the present inventionprovides the advantage of less incremental cost per any extra antennathat is added to the system. The total number of secondary frequency RFsections used in the preferred embodiment of the present invention isequal to the number of secondary frequency RF signals, whereas in priorsystems with a dedicated hardware for each antenna, the number ofsecondary frequency RF sections is equal to the number of antennas inthe system, which may be very large depending upon the application. Thisis achieved by multiplexing all the secondary frequency RF signals,thereby using only one secondary frequency RF section for all thesecondary frequency RF signals.

Yet another advantage of the preferred embodiment of the presentinvention enhanced accuracy for attitude processing systems by virtue ofreal time calibration of RF path biases.

Alterations to the Preferred Embodiment

Some variations of the preferred embodiment of the present invention arediscussed hereinafter. For civilian applications, the dedicated L1 RFsections commonly have narrow-band RF sections with a bandwidth of 2MHz. Narrow-band RF sections have sufficient bandwidth to capture mostof the power in the L1 C/A code signal but cannot capture the L1 Y codesignal. For example, a Zarlink GP2015 chip may be used as a narrow-bandL1 RF section. Civilian applications with no knowledge about encryptionof the P code employ cross-correlation using the L1 RF signals. Hence,to capture the Y code of the L1 RF signal, a dual frequency L1/L2RFsection is used with two 20 MHz band-pass filters (one centered at theL1 RF frequency and the other one at the L2 RF frequency) for capturingthe 10 MHz Y code modulated on both the L1 and the L2 RF signals.

In another variation of the preferred embodiment (for civilianapplications), a wide-band 20 MHz L1 RF section is used. In this case,only a 20 MHz L2 RF section is required for capturing the L2 RF signal.

In yet another variation of the preferred embodiment (for militaryapplications with known encryption of the L2 RF signal), a singlewideband 20 MHz L2 RF section at the output of the multiplexer is used.In this case, the L1 RF section may also be a wide band 20 MHz L1 RFsection. Although military receivers do not perform cross-correlation ofthe L2 RF signals, typically, the Y code on both the L1 and the L2 RFsignals is tracked using wide band RF sections. This is done to preventjamming and for anti-spoofing purposes.

In still another variation of the preferred embodiment, K of the Msecondary frequency RF sections operate at the same secondary RFfrequency. Therefore, K secondary frequency RF sections processsecondary frequency RF signals from K antennas in parallel i.e.simultaneously. By way of an example, assume that N=4, K=2, the primaryfrequency is L1, the secondary frequency is L2, where N is the totalnumber of antennas. This implies that there are 4 L1 RF sections and twoL2 RF sections. In this case, the measurement system can simultaneouslytrack L1 RF signals from 4 antennas and L2 RF signals from 2 antennas.This improves the accuracy of the system.

An Alternate Embodiment of the Present Invention

Hereinafter, an alternate embodiment of the present invention isdescribed. The alternate embodiment of the measurement system comprisesa plurality of antennas receiving RF signals from a plurality of signalgenerating sources. Each RF signal consists of a primary frequency RFsignal and at least one secondary frequency RF signal.

An array of dedicated RF splitters is used to split these RF signalsinto a plurality of output signals. The RF splitters split the RFsignals into a first output signal and at least one second outputsignal; the number of second output signals being dependent on thenumber of secondary frequencies present in the RF signals.

The first output signals from all the antennas are filtered, downconverted and sampled to obtain sampled primary signals using a primaryfrequency RF section dedicated to each of the antennas. Hence, everyantenna has one RF splitter and one primary frequency RF sectionassociated with it.

Each primary frequency RF section is connected to a separate firstcorrelator. The sampled primary signals are passed on to thecorresponding first correlator so that code and carrier phases for theprimary frequency RF signals may be resolved.

One stream of the second output signal from each of the antennas is fedto every multiplexer (there is a separate multiplexer associated witheach RF splitter), i.e. each RF splitter sends a second output signal toevery multiplexer. The multiplexer switches between all the secondoutput signals from all the antennas. Each multiplexer is connected tojust one secondary frequency RF section. Hence, each secondary frequencyRF section receives second output signals from an antenna for a fractionof time.

Each secondary frequency RF section filters the second output signal toderive a secondary frequency RF signal, which is then down converted andsampled. The number of multiplexers and secondary frequency RF sectionsis the same as the number of secondary frequencies present in the RFsignals received by the antennas.

All the sampled secondary signals are sent to a second correlator, whichis connected to all the secondary frequency RF sections. In yet anotherembodiment of the present invention, the second correlator is connectedto all the secondary frequency RF sections through buffers that arecontrolled by the processor. The second correlator derives code andcarrier phases for all the secondary frequency RF signals, which arethen passed on to the processor.

The processor resolves carrier cycle ambiguities for the primaryfrequency RF signals employing any of the aforementioned algorithms: theintra-platform parallel processing algorithm, the intra-platformsequential processing algorithm, the inter-platform parallel processingalgorithm and the inter-platform sequential processing algorithm. Themeasurement system uses the last two algorithms, whenever it isreceiving communication from the reference base station. The processorcalculates the position of all the antennas using code and phasemeasurements for the primary frequency RF signals. The processor usesthe position information of the antennas to continuously track theobject.

The processor also calibrates the RF path biases for the primaryfrequency RF sections using the RF path bias calibration algorithm.

All the applications and advantages of the preferred embodiment areapplicable to the alternate embodiment as well.

A Block Diagram in Accordance with the Alternate Embodiment

Referring now primarily to FIG. 2, an exemplary block diagram of themeasurement system in accordance with an alternate embodiment of thepresent invention is illustrated. The alternate embodiment differs fromthe preferred embodiment in the manner in which the correlated L1signals are fed to a second correlator 222 for cross-correlation of theL2 RF signals. In the alternate embodiment, the correlated L1 signalsare fed to second correlator 222 through a processor 214 forcross-correlation of down converted and sampled L2 RF signals. On thecontrary, in the preferred embodiment, correlation of both the RFsignals is performed by correlator 124.

Henceforth, the alternative embodiment illustrated by FIG. 2 shall bedescribed with reference to the antenna 202 a. The description providedwith respect to antenna 202 a should be construed for antennas 202 b and202 c as well; elements such as splitters, first output signal, secondoutput signal, L1 RF sections and L2 RF section referred with respect toantenna 202 a should be construed as corresponding elements for antennas202 b and 202 c as well.

Antennas 202 a, 202 b and 202 c receive multiple frequency signals frommultiple GPS satellites. Input signals received by antenna 202 a areamplified, filtered and sent to an RF splitter 204 a dedicated toantenna 202 a. RF splitter 204 a outputs a first output signal 206 andtwo second output signals i.e. second output signal 208 a and secondoutput signal 208 b. The first output signal is the signal that is fedto a primary frequency RF section. The second output signal is thesignal that is fed to a multiplexer for subsequent processing ofsecondary frequency RF signals. All the output signals of RF splitters204 a through 204 c include the L1 frequency signals and the L2frequency signals. First output signal 206 is fed to an L1 RF section210 a connected to RF splitter 204 a. L1 RF section 210 a is dedicatedto the L1 RF signals received from antenna 202 a. Antenna 202 b has anL1 RF section 210 b dedicated to it and antenna 202 c has an L1 section210 c dedicated to it.

L1 RF section 210 a filters the first output signal 206 and receivesonly the L1 signal. Similar filtering is performed by L1 RF section 210b and L1 RF section 210 c on first output signal to obtain the L1 RFsignal. The L1 RF signal is thereafter down converted from RF to an IFand sampled. IF down conversion and sampling for each of L1 RF sections210 a through 210 c is phase locked to an oscillator 209. The L1 sampledsignals are sent to corresponding first correlators 212 a through 212 c,which correlate the input L1 RF signal with the C/A code. Firstcorrelators 212 a, 212 b and 212 c output a set of in-phase andquadrature signals that are sent to a processor 214 for furtherprocessing.

Second output signal 208 a moves along a second path from RF splitter204 a to be fed to a multiplexer 216 a for multiplexing. Multiplexer 216a switches between the second output signals from RF splitters 204 athrough 204 c. Thus, multiplexer 216 a receives one of the second outputsignals from each of the RF splitters 204 a through 204 c. Multiplexer216 a switches between second output signal 208 a received from RFsplitter 204 a, the second output signal received from RF splitter 204 band the second output signal received from RF splitter 204 c for apre-configured fraction of time. Multiplexer 216 a is dedicated to an L2RF section 218.

Second output signal 208 b moves along a third path from RF splitter 204a to be fed to another multiplexer 220 for multiplexing. Multiplexer 216b is dedicated to an L5 RF section 220. Multiplexer 216 b receives theother second output signal from each of the RF splitters 204 a through204 c. Multiplexer 216 b switches between second output signal 208 breceived from RF splitter 204 a, the second output signal received fromRF splitter 204 b and the second output signal received from RF splitter204 c for a pre-configured fraction of time.

L2 RF section 218 and L5 RF section 220 filter, down convert and samplethe L2 RF signal and the L5 RF signal, respectively. L2 RF section 218outputs sampled L2 RF signals. Similarly, L5 RF section 220 outputssampled L5 RF signals. The sampled L2 RF signals and the sampled L5 RFsignals are stored in a buffer 221 a and a buffer 221 b, respectively.Buffers 221 a and 221 b are embodied on a RAM. The sampled L2 RF signalsstored in buffer 221 a are fed to a second correlator 222 upon commandfrom processor 214. Second correlator 222 performs cross-correlationbetween the sampled L2 RF signals and the tracked L1 phases receivedfrom processor 214. The cross-correlation of the sampled L2 RF signalsis performed to derive code and carrier phases of the L2 RF signals.Similarly, code and carrier phases of the L5 RF signals are derived bythe cross-correlation of the sampled L5 RF signals and the tracked L1phases. The cross-correlation of the L2 RF signals is carried out usingany of the standard cross-correlation algorithms known in the art.Second correlator 222 provides the initial code and the initial carrierphase for the L2 RF signals to processor 214 for reconstructing the L2RF signals. The initial code and the initial carrier phase refer to thecode and the carrier of the L2 RF signal, which is obtained for theavailable L2 RF signal (where some part of the L2 RF signal remainsuntracked due to multiplexing). Second correlator 222 is embodied on anFPGA.

Processor 214 uses the correlated L1 RF signals received from firstcorrelators 212 a through 212 c and the initial code and the initialcarrier phase for the L2 RF signals from second correlator 222 fordetermining carrier cycle ambiguities for the L1 RF signals. Processor214 resolves carrier cycle ambiguities employing any of the followingalgorithms: the intra-platform parallel processing algorithm and theintra-platform sequential processing algorithm. If the measurementsystem is receiving communication from the reference station, theprocessor can use any one of the following algorithms for carrier cycleambiguity resolution: the inter-platform parallel processing algorithmor the inter-platform sequential processing algorithm. Processor 214also solves for positions of all the antennas using algorithms known inthe prior art for carrier phase measurements. Positions of the antennasthus derived are used to track the object coupled to the antenna. All L1RF sections 210 a through 210 c, L2 RF section 218, L5 RF section 220and processor 214 are phase locked to oscillator 209 for synchronousoperation.

While certain embodiments of the present invention have been illustratedand described, additional variations and modifications in theseembodiments may occur to those skilled in the art once they learn of thebasic inventive concepts. Values for various parameters mentioned in thedescription of the preferred embodiment are merely illustrative innature. Therefore, it is intended that the appended claims shall beconstrued to include both the preferred embodiment, and all suchvariations and modifications as fall within the spirit and scope of theinvention as described in the claims.

1. A measurement system for calibrating in an attitude determinationreceiver, the measurement system comprising: a plurality of antennas; aplurality of first filters, one filter for each antenna of the pluralityof antennas, each filter operable to obtain information from arespective one of the plurality of antennas at a primary frequency; asecond filter operable to connect with at least one of the plurality ofantennas and operable on signals bypassing the first filters; and aprocessor operable to calibrate biases for the information at theprimary frequency with a RF path bias calibration algorithm as afunction of an output of the second filter based on the signalsbypassing the first filters.
 2. The system of claim 1 furthercomprising: a multiplexer operable to connect each of the plurality ofantennas to the second filter.
 3. The system of claim 1 wherein thesecond filter is operable to obtain information at a secondary frequencydifferent than the primary frequency.
 4. The system of claim 1 furthercomprising a splitter operable to output to one of the first filters andthe second filter, the signals bypassing the first filters being outputfrom the splitter to the second filter.
 5. The system of claim 1 wherethe information comprises information from an orbiting satellite, andthe primary frequency comprises a navigation satellite frequency.